1. Field of the Invention
The present invention relates generally to solar collectors and, more particularly, to a cable-tensioned membrane solar collector module with variable tension control.
2. Description of the Prior Art
Recent developments in the solar heliostat collector art include a trend toward manufacturing reflector panels or modules for concentrating heliostat collector assemblies with thin, flexible, lightweight, reflector materials. Examples of such reflector materials are thin metallic sheets of steel or aluminum, which are often called foils. Reflector modules manufactured from these materials are commonly referred to as stretched-membrane solar collectors.
A concentrating solar collector may be simply defined as a reflector for optically collecting the sun's radiation and concentrating incident radiation at a focal area. The reflector is typically a mirror or a plurality of mirrors supported by a metal-constructed frame. Independently steered solar reflectors are generally referred to as heliostats. Solar radiation is commonly known as sunlight and, generally speaking, concerns electromagnetic radiation and photons emitted by the sun. The focal area, broadly speaking, is a point or region to which the collector reflects all incident solar radiation. Concentrating generally means increasing the intensities of solar radiation to temperatures needed for industrial process heat or thermoelectrical power stations.
To concentrate solar radiation, individual solar collectors are usually employed in an array to point or focus the radiation onto an absorber target. In most cases, the absorber target is an absorber/receiver. The absorber/receiver, which may be a cavity-type, is normally positioned at either an aimpoint or the focal area of the array, as previously suggested, to absorb maximum solar energy flux. Solar energy flux generally means energy flux transmitted from the sun, which is in the form of electromagnetic radiation. The absorbed solar energy flux is usually carried away by a suitable heat transfer fluid to provide electrical or mechanical power, to operate thermomechanical apparatus, or to provide industrial process heat.
The aforesaid trend toward producing lightweight solar collectors is dictated in part by the high manufacturing costs and heavyweight of glass/metal-type reflector panels and reflector supports. The reflector panels and support structures are often fabricated from thick, heavy metal, glass, and composite materials to meet the solar reflectivity and specularity imposed by the heliostat collector performance requirements, as well as the strength and rigidity standards imposed by the heliostat/collector survival requirements. Reflectivity is generally associated with the reflector material and specular variation in the reflection of radiant energy. Specularity is the degree to which beam radiation can be successfully reflected without scattering the light rays impinging on the reflector surface. The finish and flatness of a surface will affect its specularity. For example, silver-glassed mirrors have traditionally provided the highest reflectivity and best specularity. Metal is a favorable material for manufacturing the reflector support because it gives the reflector panel the capacity to withstand environmental loads without warping, bucking, or fracturing, which eventually could lead to failure. Examples of such environmental loads are gravity loads, wind loads, and ice/snow loads.
Unfortunately, the heavy deadweight load of the glass/metal reflectors and the reflector supports frequently produces stresses and deformation that undesirably add to the harmful stresses produced by the environmental loads. Additionally, the aforementioned use of heavy glass, metal, and other structural materials to fabricate the reflectors and their supports is one major reason for their high manufacturing costs.
In addressing the disadvantages associated with glass/metal-type reflector panels by producing lightweight stretched membrane solar collectors which greatly simplify and reduce the weight of the reflectors, a problem has developed in shaping and tensioning the stretched reflector surfaces thereof. For example, it has often been extremely difficult to shape and tension a stretched-membrane-type reflector surface so that it produces an acceptable focal spot at the absorber/receiver cavity with minimal unabsorbed surface reflected solar flux. Also, the absorber/receiver must be sufficiently small to minimize the associated radiant and convection energy losses. Radiant and convection losses concern solar energy that is lost by the absorber/receiver after the solar radiation is absorbed. The concepts of the required focal spot size and the radiant and convection losses become even more significant when it is realized that the characteristics of a stretched membrane reflector surface and a focus provided thereby may be used to reduce radient and connection losses.
A stretched reflector surface will generally have a gravity-induced focal length which is a function of the surface tension and a reflector elevation angle. Normally, increasing the tension of the stretched reflector surface increases the gravity-induced focal length. The ideal focal length is equal to a slant range from the reflector to the absorber/receiver cavity. Hence, each solar collector in the field will have a different focal length and a different associated tension to control the gravity-induced focus. Thus, it is evident that the aforesaid reflector characteristics can be used to enhance collector system performance by reducing the size of the image at the receiver and therefore the amount of energy spillover.
Another problem is that the reflector surfaces of stretched-membrane solar collectors usually have to be tensioned and assembled at the manufacturing facilities rather than at the collector sites. They also usually require skilled workmen to assemble them. Moreover, once the collectors are factory-assembled, their focus or aimpoint is usually not easily adjustable; therefore it can be difficult to produce various concentration ratios to meet specific collector site requirements. Concentration ratios are the ratio of the intensity of solar light impinging on the absorber/receiver to the solar light impinging on the reflector surface. Notably, these ratios may be as small as one for no concentration using a single collector to as high as several thousand using a large field of collectors.
Besides, many factory-assembled collectors which do not provide a means for immediately adjusting the reflector tension during periods of operation at the collector site frequently are incapable of compensating incapable reflector tension variations and reflector deformation because of long-term reflector creep and environmental loads. Reflector creep may be defined as a slow change in reflector tension as a result of prolonged exposure to temperature excursions and environmental loads.
Still another problem related to factory-assembled collectors is that shipping constraints usually limit the size of the reflector module which can be transported to the collector site from the factory. Another problem is that most current membrane solar collectors require fairly sophisticated designs to provide the reflector surfaces with the desired durability and optics.
To cope with the aforesaid problems, the reflector surfaces of some solar collectors have been designed by tensioning a sheet of aluminized Mylar over a plurality of elongated supporting members. The supporting members impart a catenary configuration to the aluminized sheet. A prior art patent relating to such a design is U.S. Pat. Ser. No. 4,173,397. Unfortunately, however, this prior art design as well as others have suffered from one or more shortcomings. For example, this earlier design is unduly complex, comprises a number of component parts, and has a focus that is not easily controllable.
Some prior art designs have stretched a sheet of aluminized Mylar over a top of a hollow cylinder and reduced a pressure therein between to provide a desired surface configuration. An example of this design is disclosed in U.S. Pat. Ser. No. 4,288,146. Unfortunately, this design may develop leaks and changes in the pressure within the cylinder. Such leaks may, in turn, lead to undesirable and irreversible degradation of the collector focus. It will be noted that the use of a vacuum pump to maintain the desired pressure has to some degree been helpful in reducing some aspects of the pressure leakage problem. However, such a pump is an additional cost element and is power consuming. Moreover, such a vacuum system adds complexity to the collector system, requires additional maintenance and reduces system reliability.
Another prior art design somewhat similar to the design of the present invention is taught in U.S. Pat. Ser. No. 4,251,135. Here, a solar reflector having a flexible triangular reflective membrane with three sides thereabout employs a tension cable to place the membrane under tension. This design, however, fails to provide a means for adjustably varying the tension of the assembled reflector panel at the collector site. Thus, this design suffers from the same long-standing problem discussed above in connection with factory-assembled, stretched-membrane collectors that the present cable tensioned membrane solar collector module invention with variable tension control has satisfactorily overcome this problem.